1. Technical Field
The invention relates to a device for radiometrically gauging the surface of a measurement object that includes effect pigments or flakes embedded in a substrate material.
2. Background Art
Surface coatings comprising embedded flake-shaped effect pigments (flakes) are known to produce a sparkle effect—see for example the reference documents Wissling 2006 and Pfaff 2009. Flake-like effect pigments act as tiny mirrors in the substrate or medium in which they are embedded and reflect incident light. Sparkles become visible when a coating material containing effect pigments is illuminated with directional light, for example, sunlight or light from a point light source. The corresponding visual effect is a pattern of bright light sources which exhibit a high contrast and are superimposed onto the background hue of the coating material. The visual appearance of the sparkle effect is described in more detail in the reference document Kirchner 2007. The terms “appearance”, “flake” and “sparkle” have established themselves generally in the relevant specialist circles.
Depending on the type and size of the effect pigments, the sparkles produced by them will have a different brightness and colour distribution. The sparkles of pure-aluminium-based flakes, for example, are perceived as (chromatically) neutral, while other special effect pigments such as, for example, Xirallic Crystal Silver (a trademark of the company Merck KGaA) produce a sparkle pattern which exhibits a broader colour distribution.
Sparkles represent a location-dependent appearance phenomenon which requires (digital) image data of the material surface in order to be characterised. The present invention deals generally with capturing and/or measuring such image data and with the measuring means required for this purpose. More specifically, the invention deals with measurement-technological preconditions and methods for radiometrically gauging sparkles in an image.
One basic aim of every measurement device of the generic type is to capture measurement data which match the appearance of the measurement object, i.e., its visual perception. A visual assessment is typically made under defined observation conditions which include the type of light, illumination intensity, sample size, viewing and illumination geometry and the viewing distance from the sample. Favourable preconditions for a high correlation between the visual appearance and the visualisation of measurement-technological results are achieved if the measurement geometry used and the fundamental measuring technique correspond as far as possible to the observation conditions.
Favourable observation conditions for an accurate appearance assessment require a sample to be illuminated with a sufficient illumination intensity, such that the eye is accommodated and the pupil diameter is typically smaller than 4 mm. Typical observation distances between the eye and the sample are in the range of 250 to 500 mm. The aperture angle of the eye and of the light source are important parameters. They regulate the contrast, number, density and chromaticity of the sparkles which can be visually perceived.
Against this background, it is an object of the invention to provide a device for radiometrically gauging the surface of a measurement object, which is suitable for measuring sparkles and which is optimised in measurement-technological terms for this purpose, wherein the measurement device is in particular intended to be able to provide measurement data which, when visualised, match the visual perception of an observer. Another aim of the invention is to capture device-independent and technology-independent calibrated multispectral measurement data which can be used as a basis for calculating texture scales and for data exchange and communication.